A pressure sensitive adhesive, generally, is a material which adheres to a surface with slight pressure and releases from the surface with negligible transfer of the material to the surface. Silicone pressure sensitive adhesives that are known in the art are typically solvent-based adhesives. The solvents are employed primarily to reduce the viscosity of the silicone pressure sensitive adhesive to a viscosity which is easily coated onto the substrate of choice, and the solvents are removed after coating. As with any solvent-based pressure sensitive adhesive (PSA), special precautions must be taken to contain and avoid environmental exposure of the solvents and avoid flammable and explosive conditions as many of the solvents used are flammable.
Hot-melt pressure sensitive adhesives are those adhesives, which upon heating, melt to viscosities suitable for coating, but when cooled are generally in a flowless state. Hot-melt pressure sensitive adhesives exhibit the following advantages over solvent-based pressure sensitive adhesives. Hot-melt pressure sensitive adhesives: (1) do not require removal and containment of solvents; (2) due to the absence of flammable solvents, do not require special precautions to avoid fires; (3) make available coating processes other than those commonly used with solvent-based pressure sensitive adhesives; and (4) are more easily coated into thick sections with minimal bubbling, a problem which often results with coating out solvent based PSA's. In addition, hot-melt PSA's have the advantage of not containing solvents which sometimes interfere with the addition of other ingredients to the PSA.
Silicone pressure sensitive adhesives are preferred over other types of PSA's in many applications, especially in the medical area. For example, because silicone pressure sensitive adhesives are acceptable for topical use, these have found use in transdermal drug delivery applications which involve the adherence of a drug-containing patch to a patient's skin.
U.S. Pat. No. 4,865,920 to Randall P. Sweet, discloses a method of making hot-melt silicone pressure sensitive adhesives which have the inherent benefits of being composed of silicone and being a hot-melt PSA. In U.S. Pat. No. 4,865,920, the hot-melt silicone pressure adhesive composition consists of (i) a silicate resin; (ii) a silicone fluid; and (iii) 1 to 10 weight percent, based on the total weight of the silicate resin and silicone fluid, of an ester having the formula: R--C(O)OR' wherein R is a monovalent hydrocarbon radical having from 2 to 32 carbon atoms and R' is a monovalent hydrocarbon radical having from 1 to 14 carbon atoms.
Although this silicone pressure sensitive adhesive composition has been found to be highly efficacious, it is desirable to include organic waxes that decrease viscosity and improve of the coatability of the hot-melt adhesive at temperatures at or below 200.degree. C.
It is desirable that the adhesive be compatible with a variety of release liners. The new hot-melt silicone pressure sensitive adhesive must allow permeation of lipophilic drugs through the PSA. Also, the adhesive should have controllable adhesion, so that the aggressiveness of adhesion can be tailored to the application.